Kinetics and mechanisms of heterogeneous reaction of NO2 on

Atmos. Chem. Phys., 10, 463–474, 2010 www.atmos-chem-phys.net/10/463/2010/ © Author(s) 2010. This work is distributed under the Creative Commons Attribution 3.0 License.

Atmospheric Chemistry and Physics

Kinetics and mechanisms of heterogeneous reaction of NO2 on CaCO3 surfaces under dry and wet conditions H. J. Li, T. Zhu, D. F. Zhao, Z. F. Zhang, and Z. M. Chen State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China Received: 7 January 2009 – Published in Atmos. Chem. Phys. Discuss.: 17 March 2009 Revised: 15 December 2009 – Accepted: 16 December 2009 – Published: 20 January 2010

Abstract. With increasing NO2 concentration in the troposphere, the importance of NO2 reaction with mineral dust in the atmosphere needs to be evaluated. Until now, little is known about the reaction of NO2 with CaCO3 . In this study, the heterogeneous reaction of NO2 on the surface of CaCO3 particles was investigated at 296 K and NO2 concentrations between 4.58×1015 molecules cm−3 to 1.68×1016 molecules cm−3 , using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), under wet and dry conditions. Nitrate formation was observed under both conditions, while nitrite was observed under wet conditions, indicating the reaction of NO2 on the CaCO3 surface produced nitrate and probably nitrous acid (HONO). Relative humidity (RH) influences both the initial uptake coefficient and the reaction mechanism. At low RH, surface −OH is formed through dissociation of the surface adsorbed water via oxygen vacancy, thus determining the reaction order. As RH increases, water starts to condense on the surface and the gas-liquid reaction of NO2 with the condensed water begins. With high enough RH (>52% in our experiment), the gas-liquid reaction of NO2 with condensed water becomes dominant, forming HNO3 and HONO. The initial uptake coefficient γ0 was determined to be (4.25±1.18)×10−9 under dry conditions and up to (6.56±0.34)×10−8 under wet conditions. These results suggest that the reaction of NO2 on CaCO3 particle

Correspondence to: T. Zhu ([email protected])

is unable to compete with that of HNO3 in the atmosphere. Further studies at lower NO2 concentrations and with a more accurate assessment of the surface area for calculating the uptake coefficient of the reaction of NO2 on CaCO3 particle and to examine its importance as a source of HONO in the atmosphere are needed.

1

Introduction

It has been estimated that 1000–3000 Tg of mineral aerosols are emitted into the atmosphere annually (Jonas et al., 1995). The main components of mineral dust include quartz, feldspar, carbonate (e.g. calcite, dolomite) and clay. Each of these components provides surfaces and reactants for heterogeneous reactions in the atmosphere. Many gas phase species in the atmosphere could also condense or adsorb onto mineral dust during long-range transport to impact atmospheric chemistry and climate change (Chen, 1985; Quan, 1993; Carmichael et al., 1996; Zhang et al., 2000). Modeling studies suggested that approximately 40% of nitrate formation is associated with mineral aerosols (Dentener et al., 1996). Aerosol samples taken in East Asia showed a good correlation between nitrate and calcium (Zhuang et al., 1999; Song and Carmichael, 2001; Sullivan et al., 2007). In fresh Asian mineral aerosols, calcium is found primarily in the form of calcium carbonate (CaCO3 ) (Song et al., 2005; Okada et al., 2005). The observed association of nitrate with calcium in mineral dust samples after long range transport suggests that after exposure to nitrogen oxides in polluted region, calcium carbonate in mineral dust had been converted

Published by Copernicus Publications on behalf of the European Geosciences Union.

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T. Zhu et al.: Kinetics and mechanisms of heterogeneous reaction of NO2 on CaCO3 surfaces

to calcium nitrate, which in turn can change the composition and morphology of calcium-containing aerosols and enhance their hygroscopicity and the rates of heterogeneous reactions, thus influence atmospheric chemistry. Calcium nitrate can also alter the optical properties of aerosols to impact radiation, cloud formation, and global climate change. Laboratory studies have demonstrated that nitric acid (HNO3 ) can react with CaCO3 to form calcium nitrate (Fenter et al., 1995; Goodman et al., 2000; Hanisch and Crowley, 2001; Krueger et al., 2003a, b). The measured uptake coefficient accounting for the BET area of the samples was determined to be (2.5±1)×10−4 for HNO3 on CaCO3 under dry conditions (Goodman et al., 2000). Johnson et al. (2005) determined the initial uptake coefficient of HNO3 on CaCO3 to be (2±0.4)×10−3 . The net reaction probability of HNO3 on CaCO3 particles was found to increase with increasing relative humidity, from 0.003 at RH=10% to 0.21 at 80% (Liu et al., 2008a). Vlasenko et al. (2006) reported an uptake coefficient of HNO3 on CaCO3 to be 0.11 at 33% relative humidity and a humidity dependence on Arizona Test Dust. Compared to nitric acid, little is known about the reaction of NO2 on CaCO3 . The uptake coefficients of NO2 on mineral oxide particles, such as Al2 O3 , Fe2 O3 , TiO2 , have been determined to be on the order of 10−7 (Underwood et al., 1999), which is consistent with the results of the reaction of NO2 on mineral dust samples from the Cape Verde Islands (Ullerstam et al., 2003). Boerensen et al. (2000) reported the reactive uptake coefficient of NO2 on Al2 O3 at levels of 10−9 . No literature report about the uptake coefficient of NO2 on CaCO3 has been found. Considering that the concentration of NO2 in the atmosphere is at least an order of magnitude higher than that of HNO3 , the reaction of CaCO3 particles with NO2 may comprise an important atmospheric reaction that lead to the formation of calcium nitrate during the long range transport of mineral dust. This reaction could be even more important in East Asia, where industrial regions with high NO2 emissions are located in the path of long range transport of Asian Dust. To understand the importance of the heterogeneous reaction of NO2 on mineral dust, it is important to measure the uptake coefficient of NO2 on CaCO3 particles. Similar to the reaction of HNO3 on CaCO3 , the reaction of NO2 on CaCO3 may also change under the influence of surface water. Therefore, it is interesting to study the reaction mechanisms and to measure the uptake coefficient of the reaction of NO2 on CaCO3 particles under dry and wet conditions. In this study, the reactions of NO2 on CaCO3 surfaces was investigated in situ using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in combination with Xray photoelectron spectroscopy (XPS), ion chromatography (IC), and scanning electron microscopy (SEM). Initial uptake coefficients were measured and the reaction mechanisms were studied under different relative humidity (RH).

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2

Experimental

The infrared (IR) spectrum of the particle surface was investigated on line with DRIFTS during the reaction. The reactor of DRIFTS is a vacuum reaction chamber (Model HVCDR2) surrounding a Harrick Scientific diffuse reflectance accessory (Model DRA-2CS) located in the sampling compartment of a Nicolet Nexus FTIR Spectrometer equipped with a mercury cadmium telluride (MCT) detector. Data from 128 scans, 4 cm−1 resolution, taken over about 80 s were averaged for one spectrum. Besides online infrared spectroscopy measurements, for a number of selected experiments, the reaction products on the particle surface and extractable soluble components were also measured off line using X-ray photoelectron spectroscopy (XPS) and Ion Chromatography (IC), respectively. The morphological changes were observed using SEM. Further details of the instruments have been described previously (Li et al., 2006). CaCO3 (>99.999%, Alfa Aesar) powder was prepared by grinding and the size of the ground particle was about 1-10 µm with a mean value of 5.6 µm, as measured with a laser particle sizer (MasterSizer 2000, Malvern). The size and morphology of the ground CaCO3 particles was also measured with SEM (KYKY1000B, KYKY Technology Development LTD., Beijing, China) (Fig. S1: http://www.atmos-chem-phys.net/10/463/2010/ acp-10-463-2010-supplement.pdf). The ground CaCO3 particles have various shapes; to simplify the estimation of surface area, we assumed the CaCO3 particles have cubic shape. Using the size distribution measured with the laser particle sizer and using cubic shape for all CaCO3 particles, the specific geometric surface area of the CaCO3 particles (Ap) was calculated to be 0.19 m2 g−1 . The volumetric BET (Brunauer, Emmett and Teller model) surface area of particles was measured with an ASAP2010 BET apparatus (Micromeritics Co., USA). The BET surface area, ABET , was determined to be 4.91 m2 g−1 , which is 25.8 times the specific geometric surface area Ap of CaCO3 particles. For experiments with DRIFTS, about 20 mg CaCO3 particles was placed in the sample holder, pressed flat using a glass slide, and stored in a desiccator as previously described (Li et al., 2006). Water vapor was produced and gases were mixed using the apparatus displayed in Fig. S2: http://www.atmos-chem-phys.net/10/463/2010/ acp-10-463-2010-supplement.pdf. All tubes and surfaces were made of inert glass or Teflon with the exception of the stainless steel in situ reactor. Concentrations of NO2 entering the reactor were adjusted by mixing a NO2 standard gas (710 ppm in N2 , Center of National Standard Material Research) with pure N2 (>99.999%) using two mass flow controllers (FC-260, Tylan, Germany). The flow lifetime in the DRIFTS cell is 2 s. In the reactor, the NO2 diffused to the particle surface and reacted with CaCO3 . The formation of the reaction products was monitored using DRIFTS. When www.atmos-chem-phys.net/10/463/2010/


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Absorbance

0.08 we use SiO2 particles to react with NO2 as a control experi816 ment under dry conditions, we did not detect NO− formation. 0.06 3 Humidity was regulated by mixing dry nitrogen with wa100min 3520 0.04 1350 1630 748 ter vapor by bubbling through two glass grits in pure water 1043 0min 1670 1330 3610 0.02 (Millipore Corporation, Billerica, MA, USA). Humidity was 1703 1295 monitored using a temperature sensor (PT100; Vaisala, Van0.00 taa, Finland) and a humidity sensor (HM1500; Vaisala). 1793 -0.02 3640 After the reactions, the absolute numbers of nitrate and ni3140 -0.04 trite ions formed during the reaction were determined with 3800 3600 3400 3200 3000 1800 1600 1400 1200 1000 800 IC. The reacted CaCO3 particles were sonicated in 10 mL of -1 Wavenumber(cm ) pure water (Millipore Corporation), and then the filtered soFigure 1. Typical IR spectra of CaCO particles after reaction with NO under dry conditions (RH 10% (Liu et al., 2008b), this caused the broadened peak for adsorbed water in the IR spectrum. Ca(NO3 )2 (s)+nH2 O(a) Ca(NO3 )2 ·nH2 O

(R10)

Since the adsorption of NO2 was the rate-limiting step and HNO3 (a) was the intermediate product, the rate of nitrate formation in the presence of excess surface water could be expressed as the following. d{NO− 1 3} = k6 [NO2 ] dt 2

(11)

Equation (11) shows the surface nitrate formation with respect to NO2 as a first order reaction; this is consistent with the experimental result of 0.94±0.10. Under wet conditions, N2 O4 could also be involved in the reaction of NO2 with the condensed phase water, and the reaction order for loss of NO2 was reported as 1.6±0.2 (Finlayson-Pitts et al., 2003). Lee and Schwartz (1981) reported that for the NO2 reaction with liquid water, the apparent reaction order in NO2 could change from 1 to 2, depending on the reaction regime, such as phase mixed, convective mass transport, and diffusive mass transport. The presence Atmos. Chem. Phys., 10, 463–474, 2010

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of N2 O4 in solution is also a factor determining the apparent reaction order in NO2 . Under convective mass transport conditions, the NO2 reaction order should be 1. Our experiments found that the NO2 reaction order is 0.94±0.10. However, the surface/volume ratio in our experimental was about 106 m−1 , this is much higher than the 100–800 m−1 interfacial area per unit liquid volume in the study of Lee and Schwartz (1981). Thus the reaction under our experimental conditions is unlikely limited by the convective mass transport; further analysis is needed to explain the reason for reaction order of 0.94±0.10 in NO2 . In our experiments, the NO2 concentration range was narrow for reaction order determination: 4.81×1015 to 1.22×1016 molecule m−3 under dry conditions, and 4.58×1015 to 1.14×1016 molecules m−3 under wet conditions. This is unfortunately limited by the current experimental setup. With DRIFTS system the gas concentration can neither be too low due to the detection limit of the instrument nor too high because of the fast nitrate formation in the initial stage of the reaction, which may hinder obtaining exact reaction rate from the kinetic curve. However, this narrow range should not prevent us from discussing the reaction order, as long as we do not extrapolate the results outside the range of NO2 concentration. Moreover, the correlation coefficient is high in Fig. 3 (R=0.98) which implies the reaction order is reliable within the NO2 concentration range. If future experimental conditions allowed, one should expand the range of NO2 concentrations for rate order determination. 4.3

Implications for the troposphere

Under dry conditions (RH52%, with the increase of water condensed on the surface of CaCO3 particles, the γ0 of NO2 on CaCO3 particles calculated using BET surface area could have been underestimated. As significant amount of condensed water could make the surface of CaCO3 particles smoother and less pores, the available surface area for the reaction should be close to the geometric surface area of a cubic shape particle. When calculating the uptake coefficient, to use which type of surface area depends on the surface properties of the particles. For our experiment, at wet conditions and with significant amount of condensed water on the surface, we should use geometric surface area to calculate γ0 .

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Using 0.19 m2 g−1 as the specific geometric surface area of CaCO3 particle samples, from Table 1 we calculated γ0 at RH=60–71% to be (6.56± 0.34)×10−8 ; this is more than twenty times higher than the value calculated with BET surface area. The uptake coefficient of HNO3 on CaCO3 has been measured by many researchers using different methods, it was found to increase with RH (Vlasenko et al., 2006; Liu et al., 2008) and varies from 10−5 ∼10−4 (Goodman et al., 2000; Underwood et al., 2000; Johnson et al., 2005) at 0% RH to 0.01–0.1 at >20% RH. The large variation of the uptake coefficient of HNO3 on CaCO3 may due to different methods as well as the different surface areas used in calculating uptake coefficient (Fenter et al., 1995; Hanisch and Crowley, 2001). Even considering that NO2 concentrations in the atmosphere is orders of magnitude higher than that of HNO3 , our experimental results suggest that the reaction of NO2 on CaCO3 particle is unable to compete with that of HNO3 in the atmosphere. However, under wet conditions or RH>52%, in order to obtain reliable uptake coefficients, one needs to carefully choose the type the surface area and determine it accurately. Moreover, under wet conditions, the IR, XPS, and IC evidences for the formation of nitrite and its subsequent decrease on CaCO3 particle suggests the reaction of NO2 with CaCO3 could be a source of HONO in the atmosphere. 5

Conclusions

The heterogeneous reaction of NO2 with CaCO3 under dry and wet conditions produced nitrate. Under wet conditions, nitrite formation was observed with IR, XPS, and IC, indicating the reaction of NO2 on the CaCO3 surface probably produced nitrous acid (HONO). Relative humidity (RH) influenced both the initial uptake coefficient and the reaction mechanism. The initial uptake coefficient of NO2 on CaCO3 particles γ0 was found to change with increasing relative humidity. Using BET surface area, γ0 under dry conditions (RH∼0%) was determined to be (4.25±1.18)×10−9 , while under “wet” conditions (RH=60–71%), it was slightly lower, with a value of (2.54±0.13)×10−9 . Our results show that relative humidity had a major influence on the surface properties of CaCO3 particles by not only influencing the initial uptake coefficient but also by changing the reaction mechanism. At low RH, surface −OH produced via the dissociation of surface-adsorbed water by an oxygen vacancy on the CaCO3 surface was the rate determining factor in the reaction. The reaction was second order in NO2 . As RH increases, water starts to condense on the surface and the gas-liquid reaction of NO2 with the condensed water begins. With high enough RH (>52% in our experiment), the gasliquid reaction of NO2 with condensed water becomes dominant, producing HNO3 and HONO. At this stage, the reaction appeared to be first order in NO2 .

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T. Zhu et al.: Kinetics and mechanisms of heterogeneous reaction of NO2 on CaCO3 surfaces At high RH, condensed water could make the surface smoother with less pores, other than the BET surface area, the geometric surface area should be used to calculate γ0 . Using 0.19 m2 g−1 as the specific geometric surface area of CaCO3 particle samples, we calculated γ0 at RH=60–71% to be (6.56±0.34)×10−8 ; this is more than twenty times higher than that under dry conditions. Moreover, CaCO3 is alkaline and can react with NO2 to form calcium nitrate which has strong hygroscopicity and could deliquesce at RH>10%. This will help exposing CaCO3 to NO2 , until CaCO3 is fully reacted. The uptake coefficient of HNO3 on CaCO3 ranged from 10−4 to 0.1, depending on RH. Our results show that the heterogeneous reaction of NO2 on CaCO3 is unable to compete with that of HNO3 in producing calcium nitrate on mineral aerosol in the atmosphere. However, the NO2 reaction with water adsorbed on CaCO3 could be a source of HONO in the atmosphere. Further efforts to detect HONO formed in the reaction of NO2 on CaCO3 particles are necessary. Acknowledgements. This work was supported by the National Basic Research Priorities Program (Grant No. 2002CB410802) and National Natural Science Foundation of China (Grant No. 40490265, 20637020). Edited by: M. Ammann

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